Device for determining the fill level of a filling substance in a container

ABSTRACT

The invention relates to a device for determining the fill level of a filling substance in a container. The device includes a signal generator unit which generates high-frequency signals, a transmitter/receiver pair which transmits the signals via an antenna and receives the signals which have been reflected on the surface of the filling substance. A coaxial line is also included which has an inner conductor and an outer conductor for bearing the signals, and an evaluation unit which determines the fill level in the container using the transit time of the signals. The antenna has a wave duct which is delimited by a rear wall. A send line is provided on the rear wall of the wave duct, which runs substantially within the wave duct, whereby a first end section of the send line is connected to the inner conductor of the coaxial line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for determining the fill level of afilling substance in a container having a signal generating unit whichgenerates high-frequency signals, having a transmitter/receiver unit,which transmits the signals via an antenna and receives the signalsreflected from the surface of the filling substance, having a coaxialline with an inner conductor and an outer conductor for carrying thesignals, and having an evaluation unit, which from the transit time ofthe signals ascertains the fill level in the container; the antenna hasa waveguide, defined by a back wall, and a send line is provided on theback wall of the waveguide, which send line extends essentially insidethe waveguide, and a first end section of the send line is connected tothe inner conductor of the coaxial line.

2. Related Art

Transit time methods utilize the physical principle according to whichthe transit distance is equal to the product of the transit time and therate of propagation. In the case of fill level measurement, the transitdistance equals twice the spacing between the antenna and the surface ofthe filling substance. The useful echo signal, that is, the signalreflected from the surface of the filling substance, and its transittime are determined from the so-called echo function, or digitizedenvelope curve, and the envelope curve represents the amplitudes of theecho signals as a function of the spacing between the antenna and thesurface of the filling substance. The fill level itself is then obtainedfrom the difference between the known spacing between the antenna andthe bottom of the container and the spacing, determined by themeasurement, between the surface of the filling substance and theantenna. All the known methods that make it possible to measurerelatively short distances by means of reflected microwaves can be used.The best-known examples are pulse radar and frequency modulationcontinuous wave radar (FMCW radar).

In pulse radar, short microwave pulses are transmitted periodically. Inthe FMCW method, a continuous microwave is transmitted, which isperiodically frequency-modulated linearly, for instance in accordancewith a sawtooth function. The frequency of the received echo signaltherefore, compared to the frequency that the transmitted signal has atthe instant of reception, has a frequency difference, which depends onthe transit time of the echo signal. The frequency difference betweenthe transmitted signal and the received signal, which can be obtained bymixing the two signals and evaluating the Fourier spectrum of the mixedsignal, is thus equivalent to the spacing between the reflecting surfaceand the antenna. Moreover, the amplitudes of the spectral lines of thefrequency spectrum, obtained by Fourier transformation correspond to theecho amplitudes. This Fourier spectrum in this case therefore representsthe echo function.

Electromagnetic waves propagate in coaxial lines without dispersion bythe transversal-electromagnetic mode (TEM mode). This mode is thereforeespecially well suited for transporting wave packets or electromagneticwaves that have a certain frequency bandwidth. Fed-in wave packetsundergo practically no propagation in that case; in linearlyfrequency-modulated microwaves as well, a deviation in linearity islargely avoided.

For oriented transmission of electromagnetic waves by means of anantenna, modes are preferably employed whose broadcast characteristichas a pronounced forward lobe. The transverse electric 11 mode (TE₁₁mode), which is capable of propagation in round waveguides, has thisproperty. As a function of the dimensions of the antenna used as thewaveguide, a frequency range exists within which the TE₁₁ mode is theonly mode capable of propagation. Above this frequency range, highermodes, such as the TM₀₁ mode, are also capable of propagation but areless well suited to the oriented transmission of microwaves.

From German Utility Model DE-G 93 12 251.9, it has become known toincorporate the sending lobe laterally into the antenna embodied as around waveguide. A disadvantage of such an arrangement is that thelaterally positioned sending lobe generally requires an additionalhousing, for protecting the coaxial line connected to the sending lobe.This increases the diameter, compared to an arrangement in which theinputting of the microwaves takes place through a back wall of theantenna. Another disadvantage of this prior art is that because of theasymmetry of the arrangement, not only the TE₁₁ mode but higher modes aswell are excited. Higher modes, however, have a different broadcastcharacteristic and are therefore less well suited to orientedbroadcasting.

From European Patent Disclosure EP 0 821 431 A2, a device has becomeknown which is capable of generating a mode whose broadcastcharacteristic has a pronounced forward lobe. Moreover, this device canbe used over a wide frequency range. To that end, the signals are inputfrom the back side of the round waveguide acting as an antenna. Theinputting itself is done over a trapezoidal send line, which is disposedon the back wall of the antenna and extends essentially within theinterior of the antenna. One end of the send line is connected to theinner conductor of the coaxial line carrying the signals; the other endof the send line is put into electrical contact with the back wall ofthe antenna.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a device for determiningthe fill level of a filling substance in a container that isdistinguished by a simple structure.

This object is attained in that a second end section of the send line isdisposed freely and essentially parallel to the back wall of thewaveguide; and that the spacing between the second end section of thesend line and the back wall of the waveguide is essentially λ/8, where λis the wavelength of the high-frequency signals carried in thewaveguide. The symbol λ represents the wavelength of the high-frequencysignals carried in the waveguide at a frequency to be transmitted. Thisfrequency is for instance the mean frequency in the frequency spectrumof a high-frequency pulse to be transmitted, or the mean frequency of alinearly frequency-modulated FMCW transmission signal.

Because of the axial inputting, the device of the invention isdistinguished by a slender design. The inputting does not—as in the caseof lateral inputting—exceed the dimensions of the round waveguide. Itcan therefore be mounted without difficulty even on containers withnarrow openings. Moreover, because of the straight inputting, neitherangled plugs nor line angles, which would adversely affect thepropagation of the high-frequency signals, are necessary. In addition,because of the slight spacing of λ/8 between the second end section ofthe send line and the back wall, the antenna can have shorter dimensionsthan the antennas known until now. In them, the corresponding spacinghas always been given as λ/4.

In an advantageous embodiment of the device of the invention, it isprovided that the coaxial line is flush at the front with the back wallof the waveguide. As a result, the high mechanical effort and expensenecessary in the case of German Patent Disclosure DE 195 45 493 A1 forpassing the coaxial line through the back wall into the waveguide of theantenna is avoided.

An advantageous embodiment of the device of the invention provides thatthe first end section and the second end section of the send line aredisposed essentially perpendicular to one another. The advantage of thisembodiment is self-evident: The send line can be fabricated quite simplyby appropriate bending. Moreover, the disposition of a sending lobe onthe free end, as provided for instance in DE 195 45 493 A1, issuperfluous in conjunction with the device of the invention.

In a preferred refinement of the device of the invention, the send lineis disposed on a printed circuit board secured in the antenna. Thismakes producing the device of the invention substantially easier.

Moreover, an advantageous embodiment of the device of the inventionprovides a dielectric in the waveguide of the antenna, which fills thewaveguide at least in the environment of the send line. The dielectricis for instance PTFE (polytetrafluoroethylene).

DESCRIPTION OF THE DRAWINGS

In a further embodiment of the device of the invention, the antenna canbe secured to a container via a securing device, for instance via aflange or a male thread.

FIG. 1: which is a schematic illustration of the device of theinvention;

FIG. 2: which is a schematic illustration of a first embodiment of thedevice of the invention;

FIG. 3: which is an illustration, shown partly in longitudinal section,of a second advantageous embodiment of the device of the invention; and

FIG. 4: which is a section through the embodiment shown in FIG. 3, withthe section rotated 90° from the section shown in FIG. 3.

FIG. 1 shows a schematic illustration of the device of the invention. Asolid or liquid filling substance 2 is stored in a container 4. Todetermine the fill level, the fill level measuring instrument 1 of theinvention is used, which is mounted in an opening 5 in the cap of thecontainer 4. Via the antenna 10, transmission signals generated in thesignal generation and transmitter unit 6; 7, in particular microwaves,are broadcast in the direction of the surface 3 of the filling substance2. At the surface 3, the transmitted signals are partly reflected asecho signals. These echo signals are received in thereception/evaluation unit 8; 14 and evaluated. By means of thetransmission/reception shunt 9, in the example shown the transmitterunit 6 and the receiver unit 7 are decoupled from one another. If atransmitter unit 6 and a separate receiver unit 7 are used, then thetransmission/reception shunt 9 can naturally be dispensed with.

In FIG. 2, a schematic illustration of a first embodiment of the deviceof the invention can be seen. In the case shown, the coaxial line 11 isshown in detail between the transmission/reception shunt 9 and theantenna 10. The antenna comprises a waveguide 16, which is closed off onone of its face ends by a back wall.

The signals to be transmitted and the reflected signals are carried viaa coaxial line 11. This line comprises an inner conductor 12 and anouter conductor 13. The inner conductor 12 is connected to the first endsection 18 of the send line 17. To that end, a leadthrough 23, forinstance a glass leadthrough, is provided in the back wall 15. Theleadthrough 23 has an insulation layer, which assures that in the regionof the leadthrough 23, no electrically conductive connection existsbetween the inner conductor 12 and the back wall 15.

The antenna 10 is made from an electrically conductive material, such asaluminum or special steel, or from a plastic coated with a conductivematerial. The waveguide 16 of the antenna 10 has a circular crosssection and—as already noted above—is closed off on one face end by theback wall 15. The antenna 10 consequently has the geometry of ashort-circuited round waveguide.

The send line 17 is embodied as a hook bent at a right angle, and thesecond end section 19 of the send line 17 extends essentially parallelto the back wall 15 of the antenna 10. The spacing between the secondend section 19 and the back wall 15 is approximately λ/8; λ is thewavelength of the high-frequency signals, of a predetermined meanfrequency, that are carried in the waveguide 16. The send line 17 isembedded in a dielectric 22. In the case shown, the dielectric 22 fillsup the waveguide 16 of the antenna 10 completely, and moreover it istapered conically in the direction of the broadcasting of the signals.In the view shown in FIG. 2, the device of the invention is accordinglyused in conjunction with a horn antenna. It is understood that thedevice of the invention can also be used in conjunction with a rodantenna. Both embodiments additionally improve the broadcastcharacteristic of the antenna 10, but otherwise are not absolutelynecessary for the ability of the device of the invention 1 to function.

The antenna 10 also has a securing device, by means of which the deviceof the invention can be secured to a container 4. In the case shown, amale thread serves as the securing means. It is understood that themounting can also be done by means of a flange.

Instead of a send line 17 extending freely in the waveguide 16 of theantenna 10 and secured only on its first end 18, a printed circuit board20 on which the send line 17 is disposed can also be used. Thisembodiment is shown schematically in FIGS. 3 and 4. For securing theprinted circuit board 20, a groove is for instance provided in the backwall 15; further grooves are provided on the inside face of thewaveguide, at the corresponding points diametrically opposite oneanother. The electrical contact between the first end section 18 of thesend line 17 and the inner conductor 12 is made analogously to theprovision of electrical contact for the send line 17 without a printedcircuit board 20.

List of Reference Numerals

1 Device of the invention

2 Filling substance

3 Surface of the filling substance

4 Container cap

5 Opening

6 Signal generating unit

7 Transmitter unit

8 Receiver unit

9 Transmission/reception shunt

10 Antenna

11 Coaxial line

12 Inner conductor

13 Outer conductor

14 Evaluation unit

15 Back wall

16 Waveguide

17 Send line

18 First end section

19 Second end section

20 Printed circuit board

21 Securing device

22 Dielectric

23 Leadthrough

24 Hollow chamber

What is claimed is:
 1. A device for determining the fill level of afilling substance in a container, comprising: a signal generating unitwhich generates high-frequency signals; a transmitter/receiver unitconnected to said signal generating unit; an antenna connected to saidtransmitter/receiver unit for receiving and transmitting saidhigh-frequency signals, and for receiving the signals reflected from thesurface of the filing substance, said antenna having a waveguide, a backwall and a send line provided on said back wall, said send lineextending into said waveguide, and defining a first end section and asecond end section; and a coaxial line having an inner conductor and anouter conductor for carrying the signals, said inner conductor beingconnected to said first end section, wherein: said second end sectionbeing disposed freely and substantially parallel to said back wall; anda space is defined between said second end section and said back wall,said space is essentially λ/8, where λ is the wavelength of saidhigh-frequency signals generated by said signal generating unit.
 2. Thedevice as defined in claim 1, wherein: said coaxial line is flush at itsfront with said back wall.
 3. The device as defined in claim 1, wherein:said first end section and said second end section are disposedessentially perpendicular to one another.
 4. The device as defined inclaim 1, further comprising: a printed circuit board, and wherein: saidsend line is disposed on said printed circuit board secured in saidantenna.
 5. The device as defined in claim 1, wherein: said antenna hasa dielectric which fills said waveguide at least in the vicinity of saidsend line.
 6. The device as defined in claim 1, further comprising: asecuring device, wherein: said antenna is secured to the container bysaid securing device.